1. Technical Field of the Invention
The present invention relates to a semiconductor unit for power control mounted on a vehicle.
2. Background Art
Conventionally, vehicles such as electric vehicles and electric hybrid vehicles are configured to run by driving an onboard AC electric motor through DC to AC conversion of direct current from a battery to alternative current using an inverter. The inverter includes a plurality of semiconductor modules. Each semiconductor module has one or more semiconductor devices, a pair of electrode plates on both sides of the semiconductor devices, and connecting terminals connecting the semiconductor module to an external control circuit and a driving circuit. The semiconductor module generates heat by the semiconductor devices during operation thereof. To overcome this issue, a plurality of methods has been proposed for suppressing the heat generation (For example, Japanese Patent First Publication No. 2001-320005 and Japanese Patent First Publication No. 2002-95267).
A semiconductor unit described in the Japanese Patent First Publication No. 2001-320005 is sandwiched by a pair of cooling pipes in which a coolant flows. In addition, an insulating member and/or a soft thermally conductive material are inserted between the semiconductor module and the cooling pipe, and by applying pressure to the insulating member and the soft thermally conductive material, the cooling performance of the semiconductor module is increased.
On the other hand, as for an electronic cooling unit described in the Japanese Patent First Publication No. 2002-95267, a pair of electrode plates of a semiconductor is sandwiched by a pair of cooling pipes through a metal member or an insulating plate which has a linear expansion coefficient close to that of the semiconductor. Accordingly, an in-plane breakage of the semiconductor device and/or the insulating plate is avoided as well as an improvement of the cooling performance.
According to the semiconductor units described in the Japanese Patent First Publication No. 2001-320005 and Japanese Patent First Publication No. 2002-95267, stresses to the surface direction of a pair of electrode plates on both sides of a semiconductor device change depending on heating up and cooling down of the semiconductor module. For example, aluminum, which is a fairly thermally conductive material, is used for fabricating the cooling pipe in general. However, a liner expansion coefficient of aluminum is larger than that of the semiconductor module and ceramics, such as the insulating plate. Because the cooling pipes sandwiching the semiconductor module are fixed with a through-bolt made of, for example, steel, a stress to the semiconductor device located between the two electrode plates, which are sandwiched by the cooling pipes, is increased by heating up or cooling down of the semiconductor module thereof. As in the case of Japanese Patent First Publication No. 2001-320005, if the semiconductor module is molded, a force pushing up the electrode plate to the cooling pipe is added by a part of the mold resin because the linear expansion coefficient of the mold resin is larger than that of metal. Thus, repeating stresses to the semiconductor device, for example, by the heating up and cooling down, are increased, and as a result a lifetime of the semiconductor device is decreased.
If an initial setting of the stress to the electrode plate is lowered when a vehicle is at stop as a countermeasure to overcome the above issue, a fixing force of the semiconductor module is also lowered. In addition, an inverter unit is in a vibrational environment when the vehicle is moving, therefore a vibration of the loosely fixed semiconductor module is increased when the mold resin expansion caused by the heat generation of the semiconductor device is still small. Then, connecting terminals connecting the semiconductor module to an external control circuit and a driving circuit are distorted, thereby resulting in a short-circuit among the terminals or a breakage of the wire. An increase of rigidity and/or a setting up of a buffering-bend may be countermeasures for this issue. However, an additional manufacturing cost of the semiconductor unit may result by these.
In the case of a moldless semiconductor module, as described in the Japanese Patent First Publication No. 2002-95267, an issue of increased stress is further multiplied at joined parts (for example, a joined part with solder) between the semiconductor device and external connection terminals due to the vibration of the semiconductor module, and as a result, a decreased reliability will result.